Hydrogeology of Morgan Valley, Morgan County, Utah, USA

Morgan Valley in the Wasatch Range, like several other hinterland valleys, is a rural area characterized by extensive agricultural activity and increasing population. Groundwater in the unconsolidated valley-fill aquifer is Morgan Valley’s most important source of drinking water, but there is interest in establishing wells in bedrock aquifers along the valley margins. The purpose of our study is to provide tools for water-resource management and land-use planning. To accomplish this we (1) characterize the relationship of geology to groundwater occurrence and f low, with emphasis on determining the thickness of the valley-fill aquifer and the water-yielding proper- ties of the fractured-rock aquifers, (2) map recharge and discharge areas for the valley-fill aquifer, (3) develop a water budget for the drainage basin, (4) classify the groundwater quality of the valley-fill aquifer to formally identify and document the beneficial use of groundwater resources, and (5) use environmental tracer data to identify the likely sources of nitrate in groundwater. Morgan Valley is in the lower Weber River drainage basin, and is within a structural trough shared by Ogden Valley to the north. The Wasatch Range bounds Morgan Valley to the west, and consists mostly of Precambrian metamorphic rocks of the Farmington Canyon Complex. Most of the area surrounding Morgan Valley consists of Cambrian to Tertiary sandstone, siltstone, mudstone, and limestone; Tertiary tuffaceous rocks; and Quaternary alluvial, colluvial, and mass-movement deposits. Precambrian crystalline basement rocks and Paleozoic and Mesozoic sedimentary rocks crop out on both sides of Upper Weber Canyon. We measured relative gravity and elevation at approximately 350 points throughout the valley in 2009 to help delineate the subsurface structure beneath Morgan Val- ley. We used these data and well data to estimate the approximate thickness of the valley-fill aquifer, define the geometry of the valley fill, and locate major concealed faults. The thickness of valley-fill material is greatest in central Morgan Valley, near the towns of Morgan and Enterprise, where it is estimated to be more than 600 feet (180 m). We used 65 drillers’ logs of water wells in Morgan Valley to delineate recharge areas and discharge areas, based on the presence of confining layers and relative water levels in the principal and shallow unconfined aquifers. We mapped recharge areas to serve as a tool for protecting groundwater quality and managing potential contaminant sources in Morgan Valley. The primary recharge area for the principal aquifer system consists of uplands along the valley margins and valley-fill material with- out confining layers. Discharge areas for the unconfined aquifer in Morgan Valley occur along gaining reaches of the Weber River, but are not extensive enough to define on the map. We estimated aquifer characteristics for both the valley- fill aquifer and selected fractured-rock aquifers from existing aquifer tests and specific capacity data from drillers’ logs of water wells. Transmissivity values for the valley-fill aquifer from our data range from 6.75 to 8815 square feet per day (0.63-819 m2/d) with a median of 551 square feet per day (51 m2/d) and an average of 1340 square feet per day (125 m2/d). The areas of highest transmissivity in the valley-fill aquifer correspond to the areas having the greatest aquifer thickness. Waters yielding characteristics of fractured-rock aquifers are highly variable and depend primarily on the nature (width, amount and type of cementation, connectivity, etc.) and amount of fractures intercepted by wells completed in these aquifers. We evaluated inflow and outflow water-budget components in Morgan Valley and created a detailed water bud- get based on available climatic data, drainage patterns, land use, vegetation cover, water use, geology, soil data, and stream flow measurements. The overall total inflow to and within Morgan Valley is 661,000 acre-feet (815 hm3) per year. The overall total outflow from Morgan Valley is 600,000 acre-feet (740 hm3) per year. Many factors explain the difference between the amount of inflow and outflow, including assumptions we used to estimate these parameters based on the best available existing data. Surface-water outflow is the largest source of dis- charge, followed by evapotranspiration. Precipitation is the largest source of recharge, followed by surface-water inflow. We used water-quality data based on total-dissolved- solids (TDS) concentrations to produce a groundwater- quality classification map. We collected and analyzed groundwater from 52 water wells during spring 2004 and augmented our data with additional water samples from the Utah Department of Agriculture and Food and Utah Division of Drinking Water to create a classification map based on the Utah Division of Water Quality Board’s classification scheme. The sampled wells were selected without bias to land-use practice. We sampled 10 wells, previously sampled by the Weber-Morgan Health Department and having relatively high (>4.5 mg/L) nitrate con- centration, for nitrogen and oxygen isotopes. We used nitrate data coupled with environmental tracer data to evaluate nitrogen and oxygen isotope data to help deter- mine nitrate source(s). Nitrate likely is derived from a mixture of sources. In 2009 we sampled 2 springs and 18 wells for environ- mental tracers. Ten of these sites yield water from bed- rock and the other 10 are alluvial wells that were previously sampled in 2004. For the bedrock springs/wells, we also sampled for general chemistry (including TDS) and nitrate, but we did not use 2009 data from bedrock wells to classify the valley-fill aquifer. We sampled all 20 springs and wells for tritium, oxygen, and deuterium, and three of the bedrock wells for carbon isotopes. Average nitrate concentration for water wells in the valley fill is 2.6 mg/L. Most alluvial wells have values less than 5 mg/L. Water from three alluvial wells has nitrate values that exceed drinking water-quality standards (greater than 10 mg/L). High-nitrate concentration wells (greater than 5 mg/L) are localized and situated in recharge areas. Nitrogen and oxygen isotope data indicate that sources of nitrate include fertilizer, feed lots, cultivated and non-cultivated soils, and septic-tank systems. Total-dissolved-solids concentration for ground- water in alluvial wells ranges from 92 to 1018 mg/L, and averages 437 mg/L. Total-dissolved-solids concentration for 89% of the wells is less than 500 mg/L. Class IA (Pristine) areas are mapped throughout most of Morgan Val- ley and cover about 98% of the total valley-fill material; Class II (Drinking Water Quality) represents about 2% near Hardscrabble Canyon. All of Morgan Valley is classified as primary recharge, thus all wells were sampled in the recharge area, the area most vulnerable to contamination. The widespread agricultural activity in Morgan Valley appears to have only a minor impact on groundwater quality. The results of our study indicate the valley- fill aquifer contains mostly high-quality groundwater resources that warrant protection.